This invention relates to an infrared transducer-transmitter for non-contact temperature measurement, and more particularly, to an infrared transducer-transmitter that measures temperature without contact with the target object, utilizing infrared or visible radiation and needing only two wires for receiving power as well as transmitting its output to receiving equipment.
A vital "front-end" of modern temperature measurement systems is a transducer and associated transmitter. The transducer-transmitter (sometimes hereinafter referred to as the "instrument") is normally expected to provide a strong linear output signal, preferably on the order of 4 to 20 milliamperes D.C., in proportion to the temperature of the target object. A current signal of this type and magnitude can readily be transmitted to receiving equipment with virtually no loss of signal due to transmission line resistance.
It is believed that instruments capable of producing such desirable current signals have heretofore been exclusively of the "contact" type, that is, of the type which physically contacts the target object. The present invention, however, relates to a novel transducer-transmitter of the non-contact type, capable of producing the desirable 4 to 20 milliamperes D.C. current output signal, linearly proportional to temperature, and having the further advantage of using only two wires for its power supply and its output to receiving equipment. Because it is a remote reading device, the present invention may be used with approved thermal and pressure barriers for intrinsically safe operation in hazardous locations, and it has, moreover, no moving parts. The present invention relates to an infrared transducer-transmitter for non-contact temperature measurement which challenges in its performance and versatility conventional contact temperature measurement devices, such as thermocouples and resistance thermometers.
Accurate measurement of surface temperatures of an object without contact, utilizing radiation emitted from the object, has of course been done for many years. See, for example, U.S. Pat. No. 4,005,605, issued Feb. 1, 1977 to Donald S. Michael, and assigned to the assignee of the present application. Indeed, the principles of remote temperature measurement have been extensively used in a variety of industrial, scientific and medical applications, Typical prior art apparatus of this type consists of (a) a sensing head, (b) an electronics package remote from the sensing head and (c) a cable or transmission line interconnecting the sensing head and the electronics package. Typically, the sensing head houses optical elements for gathering and focusing radiation, a detector for receiving the gathered radiation, some electronic elements and perhaps a sighting device, and is located near the target object. The electronics package typically provides for amplification of a signal incoming from the sensing head, and processing of the signal to provide, among other things, an adjustment for emissivity, linearization of the signal and compensation for ambient temperature influences. The electronics package, which is normally installed in the control room environment, may include a variety of millivolt and milliampere outputs which enable it to be used with selected display or control equipment.
The cable or transmission line interconnects the sensing head and the electronics package. The cable in such prior art devices may have any number of conductors, from two wires to several, and typically consists of shielded instrument cable in order to insure proper operation of the overall system. Since the signal produced by the sensing head is not normally characterized by high source resistance, transmission line resistance is the limiting factor in achieving high accuracy with prior art systems when long cables are used.
In the present invention, in distinct contrast to known systems, the desired linear output signal is produced by a self-contained signal transducer-transmitter, and the need for the conventional many-conductor transmission line (with its inherent limitations) is wholly eliminated. A single unit operating from a convenient unregulated power supply, provides a versatile and highly desirable two-wire output.
One difficulty which has been encountered by prior art two-wire infrared transducer designs capable of operating on conventional power supplies providing 12 to 40 volts D.C., and of the desired 4 to 20 milliamperes D.C. output linearly proportional to sensed temperature, stems from the fact that a maximum current of 4 milliamperes at 12 volts (a total of 48 milliwatts of energy) is available for consumption by the entire transducer. With known prior art devices, in order to measure the temperature of low temperature objects (i.e., those near room temperature), the techniques of "chopping" or scanning of the incoming radiation had to be used. Also, in order to compensate for ambient temperature variation of the sensing head, a constant temperature cavity was typically provided as a reference temperature source. Both of these techniques consume power to such an extent that the minimum power needs of instruments which employ them greatly exceeds the available 48 milliwatts.
As is set forth in greater detail below, the present invention has achieved the necessary accuracy and sensitivity, yet eliminated the need for a chopper, scanner or reference cavity. Consequently, in practicing the present invention, sufficient power is available to do the necessary electronic processing of the radiation-generated signal, and to produce the preferred 4 to 20 milliamperes D.C. linear current output. Of course, other less widely used and less desirable output current spans, such as 10 to 50 milliamperes are readily applicable. 1 to 5 milliamperes is, in theory, possible with the present invention.